A developable latent image is formed in a silver halide emulsion layer of a radiographic element when it is imagewise exposed to X-radiation. Silver halide emulsions, however, more efficiently absorb and consequently are more responsive to longer (300 to 1500 nm) wavelength electromagnetic radiation than to X-radiation. Silver halide possesses native sensitivity to both the near ultraviolet and blue regions of the spectrum and can be sensitized readily to the green, red, and infrared portions of the electromagnetic spectrum.
Consequently it is an accepted practice to employ intensifying screens in combination with silver halide emulsion layers. An intensifying screen contains on a support a fluorescent phosphor layer that absorbs the X-radiation more efficiently than silver halide and emits to the adjacent silver halide emulsion layer longer wavelength electromagnetic radiation in an image pattern corresponding to that of the X-radiation received.
While the phosphor layer and emulsion layer can be integrated into one element, in most instances the adjacent silver halide emulsion layer is coated on a separate support to form a separate radiographic element. In this way, the intensifying screen, which is not permanently altered by exposure, can be reused. The most common arrangement for X-radiation exposure is to employ a dual coated radiographic element (an element with silver halide emulsion layers on opposite sides of a support), each emulsion layer being mounted adjacent a separate intensifying screen.
It has been recognized that the phosphors of highest absorption efficiencies are those in which the host compound contains at least one element from Period 6 of the Periodic Table of Elements. For example, barium sulfate, lanthanide oxyhalides and oxysulfides, yttrium tantalate, and calcium tungstate, are widely employed phosphor host compounds.
One family of phosphor host compounds that have shown promise in terms of performance, but have been little used are rare earth hafnates. L. H. Brixner, "Structural and Luminescent Properties of the Ln.sub.2 Hf.sub.2 O.sub.7 -type Rare Earth Hafnates", Mat. Res. Bull., Vol. 19, pp. 143-149, 1984, describes investigations of such phosphor host compounds. Ln is defined to include not only lanthanides, but also scandium and yttrium. A significant practical disadvantage in formulating these host phosphor compounds is that firing to temperatures in the 1800.degree. to 1900.degree. C. range is required to obtain a single phase composition. These firing temperatures render rare earth hafnates burdensome to prepare as phosphor host compounds.
One hafnium containing phosphor host compound that has been recognized to possess high efficiency in its absorption of X-radiation, but has enjoyed no practical use is optical grade hafnia. Kelsey U.S. Pat. No. 4,006,097, issued May 5, 1975, discloses to be useful in the absorption of X-radiation a phosphor satisfying the formula: EQU HfO.sub.2 :Yb
with Yb being present in a concentration of 5.times.10.sup.-3 to 1.times.10.sup.-1.
Brixner, cited above, after reporting the properties of Ti.sup.+4 as an activator for rare earth hafnates, stated:
We also looked at this same activator in pure HfO.sub.2. Under 30 kVp Mo radiation x-ray excitation, this composition also emits in a broad band centered around 477 nm as seen in FIG. 5. This emission has an intensity of about 1.6 times that of PAR CaWO.sub.4 and could therefore be of interest as an x-ray intensifying screen phosphor, especially in light of the superior absorption of HfO relative to CaWO as seen in FIG. 6. Unfortunately, the price of optical grade HfO is so prohibitive that it cannot be used in screen applications. (Emphasis added.)
Optical grade hafnia contains less than 3.times.10.sup.-4 mole of zirconia per mole of hafnia. It is the difficulty in separating zirconium and hafnium that primarily accounts for the cost of optical grade hafnia.
Zirconium and hafnium are known to be atoms of essentially similar radii, 1.454 .ANG. and 1.442 .ANG., respectively. Practically all known compounds of zirconium and hafnium correspond to the +4 oxidation state. The chemical properties of the two elements are essentially identical. The elements are found together in nature and can not be entirely separated.
Zirconia and hafnia both exist predominantly in a stable monoclinic crystalline phase at room temperature, with the size of the crystal cell being very similar. As reported by E. Iwase and S. Nishiyama, "Luminescence Spectra of Trivalent Rare Earth Ions", Proc. Intern. Sym. Mol. Struct. Spectry., Tokyo, 1962, A-407-1 to 7, the crystal lattice constants of monoclinic hafnia and zirconia are as follows:
______________________________________ Oxide a-axis b-axis c-axis .beta. ______________________________________ HfO.sub.2 5.11 5.14 5.28 99.degree. 44' ZrO.sub.2 5.21 5.26 5.375 99.degree. 55' ______________________________________
Iwase and Nishiyama investigated "high purity Hf and Zr compounds" for cathodoluminescence--i.e., fluorescence response to electron bombardment.
Kroger et al U.S. Pat. No. 2,542,336 discloses phosphors containing titanium as an activator and having a matrix comprised of one or more of the oxides of zirconium, hafnium, thorium, germanium or tin. Titanium activated zirconium oxide, magnesium stannate, calcium zirconate and zirconium phosphate are each specifically disclosed. In addition, Kroger et al suggests the formulation of phosphors which contain germanium, zirconium or hafnium and oxygen complexed with other nonmetals, such as sulfur, boron, phosphorus, silicon and the like; however, inclusions of these nonmetals as components of the host phosphor produce distinctly different crystal structures than GeO.sub.2, ZrO.sub.2 and HfO.sub.2 and are not considered relevant to this invention.
Bryan et al (I) U.S. Ser. No. 305,222, filed Feb. 3, 1989, titled X-RAY INTENSIFYING SCREEN, PHOSPHOR COMPOSITION, AND PROCESS OF PHOSPHOR PREPARATION, now U.S. Pat. No. 4,988,880, commonly assigned, discloses that efficient X-ray intensifying screens can be constructed from titanium activated hafnia phosphors containing minor amounts of zirconium, but higher amounts than found in optical grade hafnia, specifically: EQU Hf.sub.1-z Zr.sub.z
where z ranges from 4.times.10.sup.-4 to 0.3.
Bryan et al (II) U.S. Ser. No. 393,602, filed Aug. 14, 1989, titled PHOSPHOR COMPOSITION AND X-RAY INTENSIFYING SCREEN, now U.S. Pat. No. 4,988,881 commonly assigned, a continuation-in-part of U.S. Ser. No. 305,310, filed Feb. 3, 1989, now abandoned, discloses the preparation of lithium hafnate phosphors containing higher levels of zirconium than are present in optical grade hafnium sources. In the preparation of the lithium hafnate phosphor hafnia has been also formed as a secondary phase. The crystal structure of lithium hafnate is unlike that of hafnia and therefore not relevant to the present invention.